1. Field of the Invention
The present invention relates to a semiconductor memory device, and particularly to a semiconductor memory device having a complete hidden refresh function for performing data refresh without being externally instructed.
2. Description of the Background Art
In the field of portable terminals such as portable telephones, an asynchronous general-purpose static semiconductor memory device (hereinafter, referred to as “SRAM”) for which external clocks need not be supplied is widely used. Since the SRAM does not require data refresh, complex control is unnecessary. For example, it is unnecessary to control such that an access to the memory is awaited until a refresh cycle is finished. For this reason, the use of the SRAM can simplify the system configuration, and therefore, the SRAM is suitable for use with the portable terminal.
Recently, the function of the portable terminal has been improved significantly, and the terminal requires a large-capacity memory. The SRAM however has a memory cell size that is about 10 times that of a dynamic semiconductor memory device (hereinafter, referred to as “DRAM”). When a large-capacity SRAM is used, the cost for the memory chip is significantly increased, and consequently, the price of the portable terminal is increased. To overcome the problem, a new technical scheme was conceived in which, instead of the SRAM, a DRAM lower in cost per unit bit is used for the portable terminal.
The DRAM however requires complex memory control for the refresh operation. For portable-terminal manufacturers that hitherto have been engaged in design of systems using SRAMs as memories, it is not easy to use DRAMs as substitutive memories of SRAMs. Under these circumstances, many semiconductor manufacturers have begun the development of a new semiconductor memory device that is formed of a DRAM but operates as an SRAM in terms of external functions
In this new semiconductor memory device, the same memory cells as those used in the DRAM are used. On the other hand, external interfaces, such as control signals and address signals to be input to the semiconductor memory device, are the same as those to be input to the SRAM. However, different from the refresh operation or the self-refresh operation of the conventional DRAM, the refresh operation of the new semiconductor memory device is not controlled by signals received from an external source. Rather, the refresh operation is controlled by a refresh instruction signal /REFE that is periodically output from a refresh circuit provided in the semiconductor memory device (for example, see Japanese Patent Laying-Open No. 2002-352577). The new semiconductor memory device capable of such completely hidden refresh will be hereinafter referred to as a “complete hidden refresh function-equipped DRAM”. The development of the complete hidden refresh function-equipped DRAM offers support to improved functions of the portable terminals.
There is also proposed a DRAM reduced in power consumption, wherein in a self-refresh mode, an upper address is assigned to each of ways (word line groups) and a lower address is assigned to each word line belonging to each of the ways, and while one way is selected, the way selection signal for selecting the way is maintained at an active level to prevent reset of the signal to an inactive level (for example, see Japanese Patent Laying-Open No. 09-161477).
In the complete hidden refresh function-equipped DRAM as well, it is important to reduce power consumption, since it is used for a portable terminal.
With the complete hidden refresh function-equipped DRAM, however, data refresh is carried out in response to refresh instruction signal /REFE even in the active mode. This means that the read/write operation and the refresh operation co-exist in the active mode, making it impossible to fix the way selection signal to an active level. Further, the way selection signal is reset to an inactive level every time a lower address is changed even in the standby mode, for the purpose of matching with the active mode. This increases the consumed current.